buffer: always use _copy for copy

[ronag]

This fixes a performance regression for Buffer.copy(target, 0) and brings it back inline with Buffer.write.

V8 has a massive TypedArray.prototype.set penalty on SharedArrayBuffer

Buffer.set and Buffer.copy are up to 8.4x slower when writing to a SharedArrayBuffer vs a regular ArrayBuffer, while Buffer.write (string encoding) is completely unaffected.

256 bytes, varying offset (Apple M3 Pro, Node 25.6.1):
                  ArrayBuffer    SharedArrayBuffer    Slowdown
Buffer.set           13.6 ns             56.1 ns       4.1x
Buffer.copy          17.0 ns             65.1 ns       3.8x
Buffer.write         75.8 ns             74.1 ns       1.0x (unaffected)

4096 bytes, varying offset:
                  ArrayBuffer    SharedArrayBuffer    Slowdown
Buffer.set           80.3 ns            674.2 ns       8.4x
Buffer.copy          78.4 ns            677.7 ns       8.6x
Buffer.write        190.6 ns            186.1 ns       1.0x (unaffected)

[Renegade334]

If the figures from #60399 still apply for ArrayBuffer-backed buffers, is there scope to add isArrayBuffer(TypedArrayPrototypeGetBuffer(target)) to the previous conditions, and get the best of both worlds?

[mcollina]

lgtm

[ronag]

If the figures from #60399 still apply for ArrayBuffer-backed buffers, is there scope to add isArrayBuffer(TypedArrayPrototypeGetBuffer(target)) to the previous conditions, and get the best of both worlds?

doing all of these type checks also adds overhead. Furthermore the benchmark from that PR apply to concat so it's unclear what it's actual effect on Buffer.copy is...

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/71489/

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.64%. Comparing base (bcdf2e0) to head (2757712).
⚠️ Report is 383 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #62032      +/-   ##
==========================================
+ Coverage   88.53%   89.64%   +1.11%     
==========================================
  Files         704      676      -28     
  Lines      208759   206227    -2532     
  Branches    40281    39515     -766     
==========================================
+ Hits       184816   184882      +66     
+ Misses      15947    13462    -2485     
+ Partials     7996     7883     -113     

Files with missing lines	Coverage Δ
lib/buffer.js	99.15% <100.00%> (-0.85%)	⬇️

... and 298 files with indirect coverage changes

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